1. Field of the Invention
This invention generally relates to solid electrolytic capacitors that may be used for a wide variety of electronics devices, and more particularly, to a roll-formed solid electrolytic capacitor and a surface-mounted solid electrolytic capacitor having a solid electrolyte formed of conducting polymer.
2. Description of the Related Art
Recently, there are demands for enhancing capacity of an electrolytic capacitor, for downsizing the electrolytic capacitor and further for lowering impedance at higher frequencies of the electrolytic capacitor, as electronics devices are digitalized.
The solid electrolytic capacitor has an excellent frequency characteristic relative to other electrolytic capacitors, and thus attracts attentions. A chemical coat mainly formed of valve metal like aluminum or tantalum is used as an anode in the solid electrolytic capacitor. One of typical structures of the solid electrolytic capacitors having an electrode foil formed of aluminum is such that an aluminum chemical foil used for an anode having a dielectric oxide layer and an aluminum chemical foil used for a cathode are rolled together across a separator paper and are formed into a capacitor element into which monomers and an oxidizing reagent are impregnated. The capacitor element is housed in, for example, an aluminum case or a resinoid case, and is sealed.
The solid electrolytic capacitor mentioned above has a compact size and has a high capacity, and is used widely. Polypyrrole, polythiophene and polyaniline or the like is used as an electrolyte. Polyethylenedioxythiophene having a low resistivity is mainly used in order to reduce an ESR (equivalent series resistance).
The solid electrolytic capacitor mentioned above has a compact size, a high capacity and a low ESR, and is easy to form a chip, and suitable for surface mounting. In terms of the above, the solid electrolytic capacitor is fundamental to downsize electronics devices and to make the electronics devices sophisticated and low in price.
However, tanδ (dielectric loss) and the ESR are affected by the adhesive condition between the formed solid electrolyte and the cathode foil in the solid electrolytic capacitor mentioned above. In addition, in a case where the formed solid electrolyte is not dense, the contact area is reduced and the tanδ is increased. Further, the adhesiveness between the solid electrolyte and the cathode is reduced, and the ESR is increased.
In addition, in a case where a valve metal is used for the cathode, the capacity of the solid electrolytic capacitor mentioned above as a capacitor is determined from a dielectric constant of the oxide layer of the valve metal, facing area between the dielectric and the cathode, and the combined capacity of the anode and the cathode. That is, even if the cathode having high capacity is used, the combined capacity never exceeds the capacity of the anode, and an increase of the capacity is limited. In addition, in a case where a conducting polymer layer, which is dense and has a high yield point, cannot be formed, a capacitor having a high achievement ratio cannot be obtained.
In order to solve these problems, Japanese Patent Application No. 2000-114108 (hereinafter referred to as Document 1) and Japanese Patent Application No. 2000-114109 (hereinafter referred to as Document 2) disclose an arrangement in which a coated layer formed of, for example, metal nitride like TiN, TaN, NbN or a valve metal like Ti, Zr, Ta, Nb is formed on surface of a cathode foil, and the appearance ratio of capacity is thus increased. Japanese Patent Application No. 2002-299181 (hereinafter referred to as Document 3) and Japanese Patent Application No. 2004-128048 (hereinafter referred to as Document 4) disclose an arrangement in which the foil of Document 1 and Document 2 is treated, and the impedance is advanced. In addition, Japanese Patent Application No. 2001-196270 (hereinafter referred to as Document 5) discloses an arrangement in which a cathode is coated with a carbide material through vacuum evaporation method.
In addition, there is a problem that an expansion of a metal case or a closing rubber occurs and features degrades, if the solid electrolytic capacitor mentioned above is used for a surface mounted chip parts of portrait style or transverse style and is subjected to soldering at reflow.
In order to solve these problems, Japanese Patent Application No. 2002-110464 (hereinafter referred to as Document 6) discloses an art of forming a separator of resin having heat resistance that does not dissolve or does not react at high temperature with a residual oxidizing reagent after polymerizing reaction or an acid that the oxidizing reagent resolves into.
However, the arrangements disclosed in Documents 1 and 2 have a problem that a cost merit degrades. Because the metal nitride and the valve metal are expensive and there is a need for pretreating the cathode and treating the cathode during the manufacturing process.
In addition, in the art of Document 3 and 4, the anode metal is simply coated with the carbide material, and the interface resistance between the carbon and the metal is enhanced. Therefore, it is not possible to reduce the ESR and the tanδ. Further, the coated carbide material is detached easily, which causes a problem that feature degradation is promoted through long-term use.
In addition, the arrangements of Document 5 and 6 have a problem that an increase of a capacity is limited. Because reduction of density of the resin having heat resistance is limited and conducting polymer cannot be filled sufficiently.